FIG. 4 is a block diagram of a conventional digital transceiver including a receiving system 101 and a transmitting system 102.
The receiving system 101 includes an antenna terminal 42 connected to an antenna 41, an antenna switch 43 connected to the antenna terminal 42, a receiving filter 52 connected to a branch port 43a of the antenna switch 43, a high-frequency amplifier 53 connected to an output port of the receiving filter 52, a receiving mixer 54 having one input port connected to the output port of the high-frequency amplifier 53 and having the other input port connected to an output port of a phase locked-loop (PLL) oscillator 51, and an output terminal 55 connected to an output port of the receiving mixer 54. The PLL oscillator 51 is controlled with control data received through a control input port 51a. 
The transmitting system 102 includes a modulator 47 having one input port connected to the output port of a transmitter modulation input terminal 49 and having another input port connected to an output port of a local oscillator 50, a transmitting mixer 46 having one input port connected to an output port of the modulator 47 and having another input port connected to the output port of the PLL oscillator 51, a transmitting filter 45 connected to an output port of the transmitting mixer 46, and a power amplifier 44 connected between an output port of the transmitting filter 45 and a branch port 43b of the antenna switch 43.
An operation the conventional transceiver 100 will be explained.
An operation of the receiving system 101 will be explained. A high-frequency signal is received by the antenna 41 and transferred to the antenna terminal 42. The high-frequency signal is then transferred via the antenna switch 43 to the receiving filter 52 for allowing a desired signal in the signal to pass through the filter. The desired signal is amplified by the high-frequency amplifier 53. The amplified signal is mixed with an output of the PLL oscillator 51 by the receiving mixer 54. A resultant intermediate frequency signal is then released from the output terminal 55.
An operation of the transmitting system 102 will be explained. A signal output from the local oscillator 50 is modulated by the modulator 47 with a digital signal received through the transmitter modulation input terminal 49. The modulated signal from the modulator 47 is mixed with an output of the PLL oscillator 51 by the transmitting mixer 46. An output of the transmitting mixer 46 is supplied to the transmitting filter 45. An output of the transmitting filter 45 is amplified by the power amplifier 44 and transferred to the branch port 43b of the antenna switch 43. The amplified signal received by the antenna switch 43 is then transmitted through the antenna 41.
The transmitting mixer 46 mixes the output of the modulator 47 with the output of the PLL oscillator 51, and outputs signal containing a high harmonic component. Such undesired harmonic component is then eliminated by the transmitting filter 45. The conventional digital signal transceiver 100 modulates the digital signal received through the transmitter demodulation input terminal 49, thus including a number of components including the transmitting filter 45, the transmitting mixer 46, and the modulator 47.
Japanese Patent Laid-Open Publication No. 6-284037 discloses a digital transceiver including a single oscillator commonly used as the local oscillator 50 and the PLL oscillator 51.
FIG. 5 is a block diagram of another conventional digital signal transceiver 103. The transceiver 103 includes an antenna terminal 42 connected to an antenna 41, an antenna switch 43 connected to the antenna terminal 42, a receiving filter 52 connected to a branch port 43a of the antenna switch 43, a high-frequency amplifier 53 connected to an output port of the receiving filter 52, a receiving mixer 54 having one input port connected to an output port of the high-frequency amplifier 53 and having another input port connected to an output port of a frequency modulator 61, and an output terminal 55 connected to an output port of the receiving mixer 54. The frequency modulator 61 is controlled with control data received through a control input port 61c. 
The frequency modulator 61 receives a digital signal through a modulation input port 61b, receives control data through the input port 61c for controlling a PLL in the modulator and output a signal through the output port 61a. The signal output from the frequency modulator 61 is supplied to a power amplifier 44 having an output port connected to another branch port 43b of the antenna switch 43.
A high frequency signal is received through the antenna 41 and transferred to the antenna terminal 42. The high frequency signal is then transferred via the antenna switch 43 to the receiving filter 52 for allowing a desired signal in the high-frequency signal to pass through the filter. The desired signal is amplified by the high-frequency amplifier 53. The amplified signal is mixed with an output signal of the frequency modulator 61 by the receiving mixer 54. A resultant intermediate frequency signal is then output from the output terminal 55.
The frequency modulator 61 receives a digital signal through the modulation input port 61b, modulates a high-frequency signal with the digital signal, and outputs the frequency-modulated signal through the output port 61a. 